FIG. 10 is a block diagram illustrating the electrical configuration of a radio communication device as a first conventional example. This radio communication device is provided with a reference oscillator 1 for outputting the reference oscillation signal at a specific frequency, a local oscillator 2 whose oscillation frequency is variable, a mixer 3 for transmission, a quadrature modulator 5 for producing quadrature modulation on the I channel and the Q channel of a baseband signal, an antenna 6 for radio transmission and reception, a mixer 7 for reception, a signal processing circuit 8 for performing various types of signal processing on baseband (BB) transmission and reception signals, and so forth. Then, the local oscillator 2, the mixer 3 and the mixer 7 form a frequency conversion circuit.
In the case of this radio communication device transmitting a signal, initially, a baseband signal (the I channel and the Q channel) for transmission is generated in the signal processing circuit 8. Then, the generated baseband signal is subjected to quadrature modulation by means of a reference oscillation signal which is outputted from the reference oscillator 1. The signal subjected to quadrature modulation is inputted to the mixer 3 by way of a bandpass filter (BPF) and an amplifier (AMP), and then up-converted by means of a local oscillation signal which is outputted from the local oscillator 2 to become a high frequency signal to be adaptable for radio transmission. The up-converted signal then passes through another bandpass filter, another amplifier, a low-pass filter (LPF) and a duplexer to reach the antenna 6 to be outputted through radio transmission.
Then, upon reception of a signal, a signal arrived at the antenna 6 passes through the duplexer, a bandpass filter, a low noise amplifier (LNA) and another bandpass filter then to be inputted to the mixer 7. The mixer 7, also receiving a local oscillation signal from the local oscillator 2, down-converts the received signal into a baseband signal (the I channel, the Q channel). The down-converted signal passes through a low-pass filter and a baseband amplifier (BB-AMP) then to be inputted to the signal processing circuit 8.
Thus, the radio communication device of FIG. 10 generates a radio transmission signal by using the reference oscillator 1 and the frequency modulation circuit (including the local oscillator 2), and also generates a baseband reception signal from a radio signal by using the frequency modulation circuit including the local oscillator 2.
According to a first conventional example, a frequency range may be widened only by widening the frequency variable range of the local oscillator. However, because there is a trade-off between the frequency variable range of the local oscillator and the C/N deterioration and the like of an output signal, widening the frequency variable range results in deteriorating the signal purity. In addition to that, in the case of widening the frequency variable range in a single local oscillator, the response time for shifting an oscillation frequency is caused to become long. In particular, in the case of shifting from a minimum frequency to a maximum frequency, or in the case of shifting from a maximum frequency to a minimum frequency, the response time becomes long. For that reason, in order to guarantee an acceptable degree of signal purity and the response time for shifting the oscillation frequency, the frequency variable range has to be limited.
FIG. 11 is a block diagram illustrating a frequency conversion circuit as an element of a second conventional example. A radio communication device according to the second conventional example is configured almost in the same manner as that of FIG. 10 with an only difference in the configuration of the frequency conversion circuit. Specifically, the frequency conversion circuit is provided with local oscillators 2a, 2b whose oscillation frequencies are variable, the mixer 3 for transmission and a change-over switch 4. The change-over switch 4 selects one of outputs from the local oscillators 2a, 2b and then outputs it to the mixer 3. According to the second conventional example, a radio transmission signal is generated by using the reference oscillator 1 and the frequency conversion circuit (including the local oscillators 2a, 2b).
The second conventional example thus uses two local oscillators, thereby guaranteeing the response time for shifting the oscillation frequency for the respective local oscillators. However, the need of having the additional local oscillator has posed a problem of increasing the number of components as well as the scale of the circuit.